1. Field of the Invention
The present invention relates to a ceramic conduit assembly with a metal outer tube, and, more particularly, to a ceramic conduit assembly with a metal outer tube which is suitable for use in an industrial sensor such as, for example an electromagnetic flowmeter.
2. Description of the Related Art
While there have been a number of conventional techniques proposed for joining a ceramic conduit to a metal outer tube, each of the proposed conventional techniques have one or more problems.
In, for example, conventional mechanical methods such as, for example, shrink fitting, machining accuracies of the ceramic conduit and the metal outer tube for a constant interference are required. Furthermore, necessary assembly work must be completed quickly under high temperature conditions of between 300.degree. C. and 600.degree. C., thereby decreasing productivity. Moreover, changes in the working temperature within the accepted tolerance produce a space between the two members thereby lowering the air tightness of a magnetic circuit housing portion.
In, for example, mechanical methods employing an O-ring, not only are the above-mentioned machining accuracies required but changes in the working temperature leads to a deterioration of the O-ring thereby lowering the air tightness.
Conventional bonding methods using, for example, an organic or inorganic adhesive suffer from the same problems relating to deterioration due to changes in working temperature so that the strength and air tightness of the joint portions which depend upon the adhesive strength and adhesive used are adversely affected.
In conventional methods such, as for example, metallizing, complicated preparatory work must be undertaken prior to the joining process and complicated joining processes are also required, with the joining strength depending upon the bond strength of the material metallized on the ceramic surface, which bonding strength is generally weak.
Conventional electromagnetic flow meters having a ceramic conduit are disclosed in, for example, Japanese Patent Laid-Open No. 61-124,823; with this technical concept being based on the configurations of the ceramic conduit and outer tube; however, this proposed construction does not take into account any stress exerted on the joint portion between the conduit and the metal outer tube which stress would be caused by concentrated loads applied to the outer tube or low cycle fatigue caused by the temperature or pressure variations. A further disadvantage resides in the fact that a magnetic circuit housing portion thereof has poor air tightness at high temperatures or under high pressure.
In, for example, U.S. Pat. No. 4,699,310, a method for bonding a ceramic with a metal is proposed which employs a member composed of an Al alloy or pure Al as a core and an Al-Si alloy cladded on both surfaces of the core, and the member is inserted between the ceramic and the metal and is welded to both the ceramic and the metal. However, in a case where a metal outer tube is fitted on an outer peripheral portion of a ceramic conduit and is joined to the ceramic conduit, it is difficult to prevent the thermal stress that is caused by the difference in the coefficients of linear expansion between the ceramic and metal, and external loads applied to the metal outer tube from being exerted on the bonded portions. In addition, it is also difficult to apply an adequate pressing force on the fitting surfaces between the outer tube and the conduit when the conduit is joined to the outer tube through the insert members. Therefore, some problems remain regarding the joining of the ceramic conduit and the metal outer tube fitted on the outer peripheral portion of the conduit, although the technique itself is valuable as a bonding method.